Preserve `list` type of `additional_special_tokens` in `special_token_map` (#12759)

* preserve type of `additional_special_tokens` in `special_token_map`

* format

* Update src/transformers/tokenization_utils_base.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/tokenization_utils_base.py

@@ -1192,7 +1192,11 @@ class SpecialTokensMixin:
         for attr in self.SPECIAL_TOKENS_ATTRIBUTES:
             attr_value = getattr(self, "_" + attr)
             if attr_value:
-                set_attr[attr] = str(attr_value)
+                set_attr[attr] = (
+                    type(attr_value)(str(attr_value_sub) for attr_value_sub in attr_value)
+                    if isinstance(attr_value, (list, tuple))
+                    else str(attr_value)
+                )
         return set_attr
 
     @property

tests/test_tokenization_common.py

@@ -2462,6 +2462,10 @@ class TokenizerTesterMixin:
                 self.assertEqual(
                     tokenizer_r.add_special_tokens({"additional_special_tokens": ["<testtoken3>", "<testtoken4>"]}), 2
                 )
+                self.assertIn("<testtoken3>", tokenizer_r.special_tokens_map["additional_special_tokens"])
+                self.assertIsInstance(tokenizer_r.special_tokens_map["additional_special_tokens"], list)
+                self.assertGreaterEqual(len(tokenizer_r.special_tokens_map["additional_special_tokens"]), 2)
+
                 self.assertEqual(len(tokenizer_r), vocab_size + 8)
 
     def test_offsets_mapping(self):